Originally, the milky liquid obtained from rubber trees was called "latex"; now this term refers to an aqueous dispersion of polymeric substances whether they are natural or synthetic. Latexes can be made by emulsion polymerization from the monomer or by emulsification of resins.
Due to environmental and energy problems, water-based systems are becoming more and more desirable. Nowadays, latexes are widely used in adhesives, textiles, inks, plastics, coatings, photographic applications, pharmaceuticals, and paper industries. In addition, monodisperse latexes also have important applications in the fields of medical, biological, and fundamental research studies. Latexes are required to be stable during preparation, storage, formulation, and applications. The stability of latexes is dependent on the total interaction energies between the particles. This has been described by the DLVO theory.
Polystyrene latexes have been used as pigment particles in paper coating, and the coating prepared from monodisperse polystyrene latex particles of different sizes has been tested for light scattering efficiency.
It has been found that a plastic pigment of polystyrene latex can meet most of the criteria for an ideal pigment. The criteria are: (1) low specific gravity, (2) high brightness, (3) high refractive index, (4) controlled particle size, (5) easily dispersible, (6) chemically inert, (7) compatible with other pigments, (8) nonabrasive, (9) low adhesive demand, (10) high price/performance efficiency. Usually plastic pigments are white in color, and the opacity can be changed by the variation of the particle size, but no satisfactory colored plastic pigments have yet been made.
Colored latexes have been made from polymerizable dyestuffs. Colored copolymers have been suggested in latex form for coating of leather. It has also been reported that colored latexes can be made by the reaction of an aqueous dispersion of microgel with hydrogen bromide through the unsaturated double bond and then followed by a nucleophilic substitution reaction with dye-molecules (Kolthoff et al, J. Polymer Sci. 15 459 (1955)).
The chemical modifications of reactive microgels cannot be carried out in aqueous solution, but involve the use of organic solvent systems. Besides, the particle sizes of microgels are usually between 5 nm and 50 nm which are too small to be used as pigments in coating applications. One of the major problems in the preparation of highly crosslinked reactive microgels is the occurrence of agglomeration phenomena, which leads to total coagulation. For polymerizable dyestuffs, the copolymerization reactivity ratios must be considered in copolymerization with styrene in addition to the solubility problem.
Generally there are three methods described in the prior art for the incorporation of dyes into a polymer latex. In "The Applications of Synthetic Resin Emulsions" by H. Warson, Ernest Benn Ltd., London, 1974 beginning on Page 848 it is stated:
"It is possible to obtain colored copolymers, even in emulsion by copolymerizing dye-stuffs including unsaturated groups, azo dyes and anthraquinone dyes being particularly suitable. These groups may be a vinyl group on an ester, an acrylate on a base, a vinylsulfonamide derivative and so on. A range of colors are available. Various specifications quote the method of preparing the colored copolymers (G. Krehbiel (to BASF), Brit. No. 877,402 (1961); H. Wilhelm (to BASF), Brit. No. 914,354 (1961); K. H. Beyer et al (2 BASF), Brit. No. 964,757 (1964)). Graft copolymers including polymerizable dyestuffs are also known (BASF, Brit. No. 965,627 (1964)).
These colored copolymers may be used directly in emulsion form for the coating of leather, thereby avoiding the numerous difficulties which have to be overcome when the leather is dyed independently. The dyestuff monomer is present at 1-15 percent by weight of the solids content of the emulsion, and a crosslinking agent such as N-methylolmethacrylamide is preferably present. This will have the effect of chemically combining the colored copolymer with the leather during the curing process on drying at the normal elevated temperature (F. Ebel et al. (to BASF), Brit. No. 998,550 (1965); H. Wilhelm et al. (to BASF), Brit. No. 1,063,219 (1967)).
Since this type of polymerization is novel some examples will be quoted here . . . " Warson then goes on to describe in a table "emulsion polymerization with colored monomers (F. Ebel et al. (to BASF), Brit. No. 998,550 (1965))." The table lists the following polymerizable dyestuffs.
2,4,5-trichloro-4'-(N-ethyl-N-acryloylhydroxyethyl)-aminoazobenzene PA0 2,4-dichloro-4'-(N-ethyl-N-acryloylhydroxyethyl)-aminoazobenzene PA0 2-methoxy-4-nitro-4'-(N-ethyl-N-acryloyhydroxyethyl)-aminoazobenzene PA0 2-cyano-4-nitro-4'-(N-ethyl-N-acryloylhydroxyethyl)-aminoazobenzene PA0 1. simple addition of water-soluble or oil-soluble dyes; PA0 2. use of copolymerizable dyes; and PA0 3. use of dyes that act as chain transfer agents. PA0 Dw=weight average particle diameter PA0 Dn=number average particle diameter
In the polymerization, 20 parts ethyl acrylate are emulsified in 50 parts water containing 0.3 parts potassium persulfate initiator, as well as emulsifier comprising 2.0 parts 20% aqueous sodium salt of a sulfonated iso-octylphenol-polyoxyethylene adduct with 25 moles ethylene oxide and 0.24 parts 50% aqueous sodium salt of sulfonated castor oil. Another emulsion is prepared comprising 7 parts polymerizable dyestuff, 40 parts ethyl acrylate, 23 parts isobutyl acrylate, 7.5 parts acrylic acid, and 5.5 parts 45% aqueous N-methylolacrylamide in 68.5 parts water containing 8.0 parts of the same sulfonated iso-octylphenol-polyoxyethylene adduct and 0.6 parts of the same sulfonated castor oil emulsifiers. The first emulsion is heated with stirring to 80.degree. and the second emulsion is added continuously over a one-hour period; at the same time, 1.2 parts potassium persulfate initiator in 20 parts of water are added continuously in a second stream. The polymerization is continued at the same temperature for a total time of 4 hours.
Other references from the same text include page 179, "The addition of dyestuffs (in the compounding of the emulsion) as distinct from pigments is sometimes desired. If water soluble, direct addition of a concentrate is possible, but the dyestuff ion must have the same charge as the dispersant, e.g., the cationic methyl violet types should not be used with anionic surfactants. An oil soluble dye should be dissolved in a small quantity of solvent or plasticizer before addition to the emulsion."
Also, page 888 states that, "There seems to be no reason why dyestuffs should not be added to emulsion polish compositions based on polymers, just as they are added to solvent-based products, and to direct wax polish emulsions. Compatibility, especially with the emulsifier system, as well as with the organic components, would have to be studied. An oil-soluble dye, probably in the wax component of the polyethylene would seem to be the most probable method of incorporation. The plasticizer could possible be used. A water-based dye, even if acid is likely to cause difficulty, due to bleeding, when any water is poured on to the polish, although it is possible that some types might be strongly absorbed onto the alkali-soluble resin, and thus be reasonably permanent."
Finally, page 857 describes the use of the red dyestuff, Waxolin OS, an azo dye, in the polymerization mixture, where it functions as a chain transfer agent, thus incorporating dyestuff endgroups into the polymer chain.
Another reference work, "Chemical Reactions of Polymers," E. M. Fettes, editor, Interscience, New York, 1964, describes on page 284 the nitration of polystyrene and its subsequent reduction (W. E. Hanford (to E. I. du Pont de Nemours), U.S. Pat. No. 2,396,786, Mar. 19, 1946; G. B. Bachman, H. Hellman, K. R. Robinson, R. W. Finholt, E. J. Kahler, L. J. Filar, L. V. Heisey, L. L. Lewis, and D. D. Micucci, J. Org. Chem. 12, 108 (1947)) to give polyaminostyrene, which is then diazotized and coupled with phenols and amines to give dyes which are insoluble in all solvents. Similar reactions with styrene-maleic anhydride copolymers are also reported (W. O. Kenyon, L. M. Minsk, and G. P. Waugh (to Eastman Kodak), U.S. Pat. No. 2,274,551, Feb. 24, 1942)). These reactions, however, were carried out on polymers dissolved in solvents rather than on emulsion polymers. The high electrolyte concentrations needed for the nitration and reduction reactions would almost certainly either dissolve the latex copolymer or flocculate the latex.
In the same book, D. Taber, E. E. Renfrew, and H. E. Tiefenthal, Chapter XV "Fiber-Reactive Dyes", pages 1113-64, describe the dyeing of textile fibers in some detail and review (pages 1143-7) the evidence for the formation of covalent bonds between reactive dyes and fibers.
As set out above the three general methods described in the prior art for coloring latexes are:
This invention provides a different, improved method for coloring latexes.